1. Field of the Invention
This invention relates to an apparatus and method of forming and projecting high precision optical images and more particularly to a method of forming the images by writing and editing with a selective heating source, advantageously a laser, on a writable, erasable, editable electronic slide and simultaneously or sequentially projecting the images, in registration, onto a receiving surface such as a projection screen or photosensitive material, in the former case for the purpose of displaying the projected image and in the latter case for the purpose of creating a hard copy of the projected image.
2. Discussion of the Prior Art
Prior art relative to this invention relates firstly to methods for creating the slices or images to be projected and secondly to methods for projecting the slides so that certain subimages are in registration. Registration of the subimages is important for a variety of applications such as the creation of color images. The text Display Systems Engineering, edited by H. R. Luxenberg and R. L. Kuehn (1968) broadly summarizes the art prior to 1968. More recent art has been reviewed by Carbone (Large Screen Display Technology Survey, Richard M. Carbone, The MITRE Corporation, Bedford, Mass., November 1982) and Todd (Lee T. Todd, Jr., Projection Display Devices, Society for Information Display, Seminar Lecture Notes, Vol. II, Paper 8.1, May 3, 1985).
Traditionally slices or images for projection have been created photographically by exposure of a light sensitive emulsion or by means of a scribing system in which a sharp point or stylus scratches the information into an opaque coating on a glass plate or film. The photographic method is not spontaneous and does not permit real-time update with new information or simultaneous writing and viewing. It also requires apparatus for film processing as well as writing. The scribing method permits real time viewing and updates but erasure of previously scribed information is impractical and the scribing rates are relatively slow.
Projection systems in which the cathodoluminescent target of a cathode ray tube serves as an electronic slide or image source overcome the disadvantages of the photographic and scribing approaches but are limited in image brightness and resolution. The projected image brightness is limited by saturation of the phosphor output as the CRT electron beam current is increased, by phosphor burn and faceplate failure due to overheating by excessive beam currents, and by practical limits on the physical size of the optics used for image projection. Resolution is limited by the increase in focused beam spot size with increased beam current, resulting in a brightness-resolution tradeoff, and by the need for high bandwidth refresh circuits to refresh the projected images at 60 Hz or more in order to eliminate visible flicker. For example, a 4000.times.4000 picture element image refreshed at 60 Hz would require a 960 MHz data input. Even if such a high bandwidth data rate were practical, positioning of an electron beam to the positional accuracy required for such a display would require extremely expensive beam indexing built into the tube or extremely stable, i.e., expensive, analog electronics. Strong magnetic shielding would be required to minimize the effect of terrestrial magnetism and local magnetic field variations on the electron beam position. Large area CRTs with image diagonals up to about 40 inches have been constructed, but their resolution is also limited by the aforementioned constraints on data rate and beam positional accuracy. Storage CRTs have been built which overcome the need for high bandwidth .refresh, but storage CRTs have significantly lower lumen output than refreshed CRTs and are therefore of limited interest for projection systems and the direct view units must be used in rooms with subdued lighting.
The aforementioned constraints on brightness of systems incorporating CRTs with cathodoluminescent targets are overcome by a class of devices known as electronic light valves. In these electronic light valves, the reflection or transmission properties of a physical medium is spatially and temporally varied by electronic means. These electronic means may include electronic scanning of an electron or optical beam or gating of a voltage across the light valve medium by an electrode array. The light valve can then be used to control the flow of light from a light source to a receiving target. With appropriate optics to image the light valve on the receiving target, the spatial and temporal variations imposed by the electronic means on the light valve can be faithfully reproduced at the position of the receiving target. Commercial light valve systems have been introduced with capabilities to project images with information content up to about 2000 TV lines.
A new type of electronic light valve display based on a laser scanned smectic (LSS) liquid crystal and capable of storing and projecting images with significantly higher than 2000 TV line information content was described by Kahn (Frederic J. Kahn, U.S. Pat. No. 3,796,999, March 1974). Even images with lower than 2000 line information content have significantly improved definition due to the resolution of the LSS light valves. More recent developments in LSS technology have been reviewed by Dewey (A. G. Dewey, Laser-Addressed Liquid Crystal Displays, Optical engineering, May-June 1984, Vol. 23, pp. 230-240).
LSS light valves consist of a thin layer of smectic liquid crystal sandwiched between two substrates. The image is written thermally by scanning a focused laser beam across the light valve. The entire image can be erased in a small fraction of a second by applying a voltage across the smectic layer. The writing beam can be transformed into an erasing beam for local editing of the written image by applying a somewhat lower voltage than required for erase of the entire image. When such a voltage is applied only those regions which are reheated by the laser beam will be erased. Advantages of these thermal smectic light valves in addition to the high resolution, erase, and local editing features are (1) the image is stored in the liquid crystal until electrically erased, thus no image refresh is required, (2) the optical properties of the image are relatively wavelength independent; thus these light valves can be used to control light from the near uv through the ir and any part of the spectrum in between, (3) there is minimal absorption of light by the liquid crystal and associated optical elements, thus this light valve can be used to control very high intensity light sources with high optical efficiency, (4) laser absorbers can be constructed to match a wide range of laser wavelengths thereby enabling use of a wide variety of writing lasers including semiconductor lasers which are relatively economical, compact, and reliable.
Despite the significant advantages cited above, commercial application of the LSS light valves has been limited by the complex and expensive scanning mechanisms required for creating very high information content images, the inability to scan precisely in a repeatable fashion, the unavailability of a method practical for creating a high resolution full color image without using multiple scanners, the unavailability of a scan system for high resolution full color images with random access as well as raster scan capabilities, the inability to display bright full color images with moderate power light sources, and the inability to implement a practical on-screen cursor. Thus a display and imaging system capable of creating precision, very high information content, full color, random scan images with a relatively simple writing system and a relatively low power projection source is desired. An additional desirable feature is a cursor. Furthermore the same precision image writing and projection capabilities desired for creation of full color images are required for producing and registering images on photosensitive hard copy materials.